add fetchCrate function to fetch rust crates

pkgs/build-support/rust/fetchcrate.nix

···
+{ lib, fetchurl, unzip }:
+
+{ crateName
+, version
+, sha256
+, ... } @ args:
+
+lib.overrideDerivation (fetchurl ({
+
+  name = "${crateName}-${version}.tar.gz";
+  url = "https://crates.io/api/v1/crates/${crateName}/${version}/download";
+  recursiveHash = true;
+
+  downloadToTemp = true;
+
+  postFetch =
+    ''
+      export PATH=${unzip}/bin:$PATH
+
+      unpackDir="$TMPDIR/unpack"
+      mkdir "$unpackDir"
+      cd "$unpackDir"
+
+      renamed="$TMPDIR/${crateName}-${version}.tar.gz"
+      mv "$downloadedFile" "$renamed"
+      unpackFile "$renamed"
+      fn=$(cd "$unpackDir" && echo *)
+      if [ -f "$unpackDir/$fn" ]; then
+        mkdir $out
+      fi
+      mv "$unpackDir/$fn" "$out"
+    '';
+} // removeAttrs args [ "crateName" "version" ]))
+# Hackety-hack: we actually need unzip hooks, too
+(x: {nativeBuildInputs = x.nativeBuildInputs++ [unzip];})

pkgs/top-level/all-packages.nix

···
 
   fetchzip = callPackage ../build-support/fetchzip { };
 
+  fetchCrate = callPackage ../build-support/rust/fetchcrate.nix { };
+
   fetchFromGitHub = {
     owner, repo, rev, name ? "source",
     fetchSubmodules ? false, private ? false,